Dense wavelength division multiplexing (DWDM) is used for transmission with one optical fiber in combination with a group of optical wavelengths. This is a laser technology for improving a bandwidth on an existing optical fiber backbone network. An optical signal to noise ratio (OSNR) in a wavelength division multiplexing system is a key performance parameter for evaluating the wavelength division multiplexing system. The OSNR is defined as a ratio of optical signal power to noise power when an optical effective bandwidth is within 0.1 nm.
As a transmission rate of the wavelength division multiplexing system is continuously increased, the OSNR is more difficult to measure. For the optical transmission systems with the transmission rates of 2.5 Gb/s and 10 Gb/s, since a spectral width is much less than a channel spacing of an optical system, an interpolation method is generally employed to measure the OSNR, i.e. the noise power outside the spectrum is first measured, then the noise power of an adjacent wavelength is obtained by utilizing the interpolation method, and finally the optical signal to noise ratio is calculated. However, for current high-speed optical transmission systems with the transmission rates of 40 Gb/s and 100 Gb/s, since a spectral width is approximate to and even greater than a channel spacing of the optical system, the noise power among channels cannot be accurately measured in the interpolation method, and the noise power of the adjacent wavelength cannot be measured. Therefore, the detection of the optical signal to noise ratio of the high-speed optical transmission system cannot be realized by the traditional interpolation method.